STEERING NOISE

I have a 2006 Sebring Convertible with 26000 miles that's developed an intermittent steering noise. It appears only after a long, high speed freeway trip and disappears during short local trips. It's never there when I bring the car to the dealer and they can't find anything wrong. The noise is a grinding, squealing, chirping kind of sound when the steering wheel is turned to the left and the car is going slowly. I'm concerned about safety of the steering system. Maybe this has happened to other 2006 Sebring Convertibles and someone can explain what us happening.